TSHttpTxnMilestoneGet() will fetch a specific milestone timer
value for the transaction txnp. These timers are calculated during the
lifetime of a transaction and are measured in nanoseconds from the beginning of
the transaction. time is used a pointer to storage to update if the call
is successful.

A pseudo index which is set to be one more than the last valid index. This is useful for looping over the data.

The server connect times predate the transmission of the SYN
packet. That is, before a connection to the origin server is completed.

A connection attempt is resolved when no more connection related activity
remains to be done, and the connection is either established or has failed.

TS_MILESTONE_UA_CLOSE and TS_MILESTONE_SERVER_CLOSE are
updated continuously during the life of the transaction, every time there is
I/O activity. The updating stops when the corresponding connection is
closed, leaving the last I/O time as the final value.

The cache OPEN milestones time only the initial setup, the
open, not the full read or write.